Characterization of Microstructures for Heat Transfer Performance in Passive Cooling Devices

2008 ◽  
Author(s):  
Ram Ranjan ◽  
Suresh V. Garimella ◽  
Jayathi Y. Murthy
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Contact Angle ◽  
Heat Transfer Performance ◽  
Passive Cooling ◽  
Liquid Volume ◽  
Geometrical Parameters ◽  
Transfer Performance ◽  
Liquid Meniscus ◽  
Cooling Devices

The topology and geometry of microstructures play a crucial role in determining heat transfer performance in passive cooling devices such as heat pipes. It is therefore important to characterize microstructures based on their wicking performance, the thermal conduction resistance of the liquid filling the microstructure, and the thin-film characteristics of the liquid meniscus. In the present study, the free-surface shapes of the static liquid meniscus in common microstructures have been modeled using the program, Surface Evolver, for zero Bond number. Four well-defined topologies, viz., surfaces with parallel rectangular ribs, horizontal parallel cylinders, vertically aligned cylinders, and spheres (the latter two in both square and hexagonal packing arrangements), have been modeled. Non-dimensional capillary pressure, average distance of the free liquid surface from solid walls (a measure of the conduction resistance of the liquid), total exposed area and thin-film percentage of surface area of the liquid meniscus have been computed. These parameters are presented as functions of the non-dimensional geometrical parameters of the microstructures, volume of the liquid filling the structure, as well as the contact angle between the liquid and solid. Based on these non-dimensional performance parameters, the microstructure, contact angle and non-dimensional liquid volume for the best performance are identified.

A Method for Measuring Contact Angle and the Influence of Surface-Fluid Parameters on the Boiling Heat Transfer Performance

2020 ◽  
Vol 142 (9) ◽  
Author(s):  
Alex P. da Cunha ◽  
Taye S. Mogaji ◽  
Reinaldo R. de Souza ◽  
Elaine M. Cardoso
Keyword(s):  
Heat Transfer ◽  
Surface Roughness ◽  
Contact Angle ◽  
Heat Transfer Performance ◽  
Sessile Drop ◽  
Pool Boiling ◽  
Transfer Performance ◽  
Transfer Coefficients ◽  
Experimental Apparatus ◽  
Boiling Process

Abstract An experimental apparatus and a computational routine were developed and implemented in order to obtain the sessile drop images and the contact angle measurement for different fluids and surface conditions. Moreover, experimental results of heat transfer coefficients (HTCs) during pool boiling of de-ionized water (DI water), Al2O3-DI water- and Fe2O3-DI water-based nanofluids are presented in this paper. Based on these results, the effect of surface roughness and nanofluid concentration on the surface wettability, contact angle, and the heat transfer coefficient was analyzed. The experiments were performed on copper heating surfaces with different roughness values (corresponding to a smooth surface or a rough surface). The coated surfaces were produced by the nanofluid pool boiling process at two different volumetric concentrations. All surfaces were subjected to metallographic, wettability and roughness tests. For smooth surfaces, in comparison to DI water, heat transfer enhancement up to 60% is observed for both nanofluids at low concentrations. As the concentration of the nanofluid increases, the surface roughness increases and the contact angle decreases, characterizing a hydrophilic behavior. The analyses indicate that the boiling process of nanofluid leads to the deposition of a coating layer on the surface, which influences the heat transfer performance in two-phase systems.

THE INFLUENCE FACTORS ON HEAT TRANSFER PERFORMANCE OF LOOP THERMOSYPHON SYSTEM

2016 ◽  
Vol 40 (5) ◽  
pp. 947-958 ◽  
Author(s):  
Li-Chieh Hsu ◽  
Guo-Wei Wong ◽  
Kung-Ting Chen
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Performance ◽  
Influence Factors ◽  
Passive Cooling ◽  
Width Ratio ◽  
Transfer Performance ◽  
Cooling Device ◽  
Cooling Performance ◽  
System A ◽  
Enhanced Boiling

The influence factors on the heat transfer performance of a loop thermosyphon system, a passive cooling device, are studied systematically. The parameters investigated include types of enhanced boiling structure, the depth to width ratio of enhanced boiling structures, the gap of evaporator, the condenser height and the inclination of evaporator. The results show the depth to width ratio and the condenser height has positive influences on the heat transfer performance. An optimal channel gap of evaporator exists and possesses better heat transfer performance. The inclination effect of evaporator may not be favorable to heat transfer. Among those, the horizontal and 90° inclination of evaporator has better cooling performance.

scholarly journals Analysis of the Wicking and Thin-Film Evaporation Characteristics of Microstructures

2009 ◽  
Vol 131 (10) ◽  
Author(s):  
Ram Ranjan ◽  
Jayathi Y. Murthy ◽  
Suresh V. Garimella
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Contact Angle ◽  
Free Surface ◽  
Capillary Pressure ◽  
Liquid Level ◽  
Performance Parameters ◽  
Liquid Meniscus ◽  
Thin Film Evaporation ◽  
Film Evaporation

The topology and geometry of microstructures play a crucial role in determining their heat transfer performance in passive cooling devices such as heat pipes. It is therefore important to characterize microstructures based on their wicking performance, the thermal conduction resistance of the liquid filling the microstructure, and the thin-film characteristics of the liquid meniscus. In the present study, the free-surface shapes of the static liquid meniscus in common microstructures are modeled using SURFACE EVOLVER for zero Bond number. Four well-defined topologies, viz., surfaces with parallel rectangular ribs, horizontal parallel cylinders, vertically aligned cylinders, and spheres (the latter two in both square and hexagonal packing arrangements), are considered. Nondimensional capillary pressure, average distance of the liquid free-surface from solid walls (a measure of the conduction resistance of the liquid), total exposed area, and thin-film area are computed. These performance parameters are presented as functions of the nondimensional geometrical parameters characterizing the microstructures, the volume of the liquid filling the structure, and the contact angle between the liquid and solid. Based on these performance parameters, hexagonally-packed spheres on a surface are identified to be the most efficient microstructure geometry for wicking and thin-film evaporation. The solid-liquid contact angle and the nondimensional liquid volume that yield the best performance are also identified. The optimum liquid level in the wick pore that yields the highest capillary pressure and heat transfer is obtained by analyzing the variation in capillary pressure and heat transfer with liquid level and using an effective thermal resistance model for the wick.

Performance Modeling of Micropost Wicks for Micro Heat Pipes at Low Heat Fluxes

2010 ◽  
Author(s):  
Stephen Sharratt ◽  
Youngsuk Nam ◽  
Y. Sungtaek Ju
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Solid Fraction ◽  
Thermal Performance ◽  
Heat Transfer Performance ◽  
Contact Angles ◽  
Heat Fluxes ◽  
Analysis Tool ◽  
Transfer Performance ◽  
Film Area

Well defined wick microstructures comprised of hexagonally packed cylindrical posts with varying solid fraction (.227–.534) are analyzed for heat transfer performance for heat pipe applications. The equilibrium free fluid surface profile under the influence of surface tension is calculated for each wick structure wetted with water. The fluid geometry is analyzed using a numerical solver so that the thermal performance (defined as a heat transfer coefficient) of the wick evaporator can be determined. Conduction through both the solid wick and liquid are considered and resistance to evaporation at the liquid-vapor interface is included. The analysis is compared to the results of a heat transfer experiment using water with a super-hydrophilic micro-manufactured copper post array. The equilibrium meniscus assumption is shown to be valid for heat fluxes less than ∼ 30 W/cm2. For low contact angles, ∼50% of the heat transfer is shown to occur within the region where fluid layer thickness is less than 2um. Heat transfer performance is shown to be a strong function of contact angle, especially for well wetting fluids. Solid fraction is shown to not be a good predictor of thermal performance. A non-dimensional normalized thin film area is presented and is a strong indicator of thermal performance. Evaporator heat transfer coefficients greater than 20 W/cm2K are predicted for large values of normalized thin film area. The modeling methods presented can be used as a design and analysis tool for predicting the effects of microscale geometry and topology on the heat transfer performance of microstructured wicks operating at low heat fluxes.

An investigation into gravity independent oscillating heat pipes

2012 ◽  
Author(s):  
◽  
Aaron A. Hathaway
Keyword(s):  
Heat Transfer ◽  
Contact Angle ◽  
Heat Pipe ◽  
Heat Transfer Performance ◽  
Heat Pipes ◽  
Experimental Results ◽  
Transfer Performance ◽  
Channel Diameter ◽  
Inverted Position ◽  
Oscillating Motion

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] A number of oscillating heat pipes (OHPs) with unbalanced structures were investigated in an effort to develop methods enabling OHPs to be gravity independent. The unbalanced structures investigated herein include the effects of uneven turn and check valves. Two 2-D and two 3-D tubular uneven-turn OHPs, four miniature uneven-turn OHPs, and one OHP with check-valves were investigated. At the same time, the theoretical analysis of the maximum channel diameter was conducted in order to find the primary factor affecting the channel size in an OHP. A model was developed that attempts to determine the maximum channel diameter by considering the contact angle effect. It is found that the contact angle significantly affects the maximum channel diameter of an OHP. In order to verify that the uneven-turn structure can generate the oscillating motion in an OHP, a heat pipe with 3 turns in the condenser and 6 turns in the evaporator was first tested. The heat pipe with uneven turns can generate and maintain oscillating motion. When the turn number increases with 16 turns in the condenser and 20 turns in the evaporator, the heat transfer performance can be further increased. An experimental investigation of a new 3-D OHP with uneven turn design was conducted in order to further develop a gravity independent OHP. Experimental results show that the uneven turn OHP developed herein can start the oscillating motion in the negative vertical position (the evaporator being above the condenser) and demonstrate that the uneven turn OHPs can significantly reduce the effect of gravity on the heat transport capability in an OHP. Three miniature OHPs (18-turn acetone OHP, 18-turn water OHP, and 20-turn acetone OHP) were developed and tested to determine whether the uneven-turn OHPs can function in a high-g environment. Experimental results demonstrate that these miniaturized uneven turn designs are extremely capable in high gravity environments and will operate effectively in any orientation. An OHP with check valves has been successfully developed and tested to determine the check valve effect on the oscillating motion and heat transfer performance in an OHP. Experimental results show that the OHP with check valves can function well in both the inverted and vertical positions with little variation between the two positions in performance once startup occurred, while the control OHP without check valves which had the same channel layout was not able to achieve startup in the inverted position. This shows that the check valves allowed the OHP to operate in the inverted position first achieving startup and then maintain oscillating motion.

Sign in / Sign up

Export Citation Format

Share Document